Article
Reference
Search for charged Higgs bosons at LEP
L3 Collaboration
ACHARD, Pablo (Collab.), et al.
Abstract
A search for pair-produced charged Higgs bosons is performed with the L3 detector at LEP using data collected at centre-of-mass energies between 189 and 209 GeV, corresponding to an integrated luminosity of 629.4 pb−1. Decays into a charm and a strange quark or into a tau lepton and its neutrino are considered. No significant excess is observed and lower limits on the mass of the charged Higgs boson are derived at the 95% confidence level. They vary from 76.5 to 82.7 GeV, as a function of the H±→τν branching ratio.
L3 Collaboration, ACHARD, Pablo (Collab.), et al . Search for charged Higgs bosons at LEP.
Physics Letters. B , 2003, vol. 575, no. 3-4, p. 208-220
DOI : 10.1016/j.physletb.2003.09.057
Available at:
http://archive-ouverte.unige.ch/unige:42582
Disclaimer: layout of this document may differ from the published version.
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Search for charged Higgs bosons at LEP
L3 Collaboration
P. Achard
t, O. Adriani
q, M. Aguilar-Benitez
x, J. Alcaraz
x, G. Alemanni
v, J. Allaby
r, A. Aloisio
ab, M.G. Alviggi
ab, H. Anderhub
at, V.P. Andreev
f,ag, F. Anselmo
h, A. Arefiev
aa, T. Azemoon
c, T. Aziz
i, P. Bagnaia
al, A. Bajo
x, G. Baksay
y, L. Baksay
y, S.V. Baldew
b, S. Banerjee
i, Sw. Banerjee
d, A. Barczyk
at,ar, R. Barillère
r, P. Bartalini
v,
M. Basile
h, N. Batalova
aq, R. Battiston
af, A. Bay
v, F. Becattini
q, U. Becker
m, F. Behner
at, L. Bellucci
q, R. Berbeco
c, J. Berdugo
x, P. Berges
m, B. Bertucci
af, B.L. Betev
at, M. Biasini
af, M. Biglietti
ab, A. Biland
at, J.J. Blaising
d, S.C. Blyth
ah,
G.J. Bobbink
b, A. Böhm
a, L. Boldizsar
l, B. Borgia
al, S. Bottai
q, D. Bourilkov
at, M. Bourquin
t, S. Braccini
t, J.G. Branson
an, F. Brochu
d, J.D. Burger
m, W.J. Burger
af,
X.D. Cai
m, M. Capell
m, G. Cara Romeo
h, G. Carlino
ab, A. Cartacci
q, J. Casaus
x, F. Cavallari
al, N. Cavallo
ai, C. Cecchi
af, M. Cerrada
x, M. Chamizo
t, Y.H. Chang
av,
M. Chemarin
w, A. Chen
av, G. Chen
g, G.M. Chen
g, H.F. Chen
u, H.S. Chen
g, G. Chiefari
ab, L. Cifarelli
am, F. Cindolo
h, I. Clare
m, R. Clare
ak, G. Coignet
d, N. Colino
x, S. Costantini
al, B. de la Cruz
x, S. Cucciarelli
af, J.A. van Dalen
ad, R. de Asmundis
ab, P. Déglon
t, J. Debreczeni
l, A. Degré
d, K. Dehmelt
y, K. Deiters
ar,
D. della Volpe
ab, E. Delmeire
t, P. Denes
aj, F. DeNotaristefani
al, A. De Salvo
at, M. Diemoz
al, M. Dierckxsens
b, C. Dionisi
al, M. Dittmar
at, A. Doria
ab, M.T. Dova
j,5, D. Duchesneau
d, M. Duda
a, B. Echenard
t, A. Eline
r, A. El Hage
a, H. El Mamouni
w, A. Engler
ah, F.J. Eppling
m, P. Extermann
t, M.A. Falagan
x, S. Falciano
al, A. Favara
ae,
J. Fay
w, O. Fedin
ag, M. Felcini
at, T. Ferguson
ah, H. Fesefeldt
a, E. Fiandrini
af, J.H. Field
t, F. Filthaut
ad, P.H. Fisher
m, W. Fisher
aj, I. Fisk
an, G. Forconi
m, K. Freudenreich
at, C. Furetta
z, Yu. Galaktionov
aa,m, S.N. Ganguli
i, P. Garcia-Abia
x,
M. Gataullin
ae, S. Gentile
al, S. Giagu
al, Z.F. Gong
u, G. Grenier
w, O. Grimm
at, M.W. Gruenewald
p, M. Guida
am, R. van Gulik
b, V.K. Gupta
aj, A. Gurtu
i, L.J. Gutay
aq,
D. Haas
e, D. Hatzifotiadou
h, T. Hebbeker
a, A. Hervé
r, J. Hirschfelder
ah, H. Hofer
at, M. Hohlmann
y, G. Holzner
at, S.R. Hou
av, Y. Hu
ad, B.N. Jin
g, L.W. Jones
c, P. de Jong
b, I. Josa-Mutuberría
x, D. Käfer
a, M. Kaur
n, M.N. Kienzle-Focacci
t, J.K. Kim
ap, J. Kirkby
r, W. Kittel
ad, A. Klimentov
m,aa, A.C. König
ad, M. Kopal
aq,
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V. Koutsenko
m,aa, M. Kräber
at, R.W. Kraemer
ah, A. Krüger
as, A. Kunin
m, P. Ladron de Guevara
x, I. Laktineh
w, G. Landi
q, M. Lebeau
r, A. Lebedev
m, P. Lebrun
w, P. Lecomte
at, P. Lecoq
r, P. Le Coultre
at, J.M. Le Goff
r, R. Leiste
as, M. Levtchenko
z, P. Levtchenko
ag, C. Li
u, S. Likhoded
as, C.H. Lin
av, W.T. Lin
av, F.L. Linde
b, L. Lista
ab, Z.A. Liu
g, W. Lohmann
as, E. Longo
al, Y.S. Lu
g, C. Luci
al,
L. Luminari
al, W. Lustermann
at, W.G. Ma
u, L. Malgeri
t, A. Malinin
aa, C. Maña
x, J. Mans
aj, J.P. Martin
w, F. Marzano
al, K. Mazumdar
i, R.R. McNeil
f, S. Mele
r,ab, L. Merola
ab, M. Meschini
q, W.J. Metzger
ad, A. Mihul
k, H. Milcent
r, G. Mirabelli
al, J. Mnich
a, G.B. Mohanty
i, G.S. Muanza
w, A.J.M. Muijs
b, B. Musicar
an, M. Musy
al, S. Nagy
o, S. Natale
t, M. Napolitano
ab, F. Nessi-Tedaldi
at, H. Newman
ae, A. Nisati
al,
T. Novak
ad, H. Nowak
as, R. Ofierzynski
at, G. Organtini
al, I. Pal
aqC. Palomares
x, P. Paolucci
ab, R. Paramatti
al, G. Passaleva
q, S. Patricelli
ab, T. Paul
j, M. Pauluzzi
af,
C. Paus
m, F. Pauss
at, M. Pedace
al, S. Pensotti
z, D. Perret-Gallix
d, B. Petersen
ad, D. Piccolo
ab, F. Pierella
h, M. Pioppi
af, P.A. Piroué
aj, E. Pistolesi
z, V. Plyaskin
aa, M. Pohl
t, V. Pojidaev
q, J. Pothier
r, D. Prokofiev
ag, J. Quartieri
am, G. Rahal-Callot
at,
M.A. Rahaman
i, P. Raics
o, N. Raja
i, R. Ramelli
at, P.G. Rancoita
z, R. Ranieri
q, A. Raspereza
as, P. Razis
ac, D. Ren
at, M. Rescigno
al, S. Reucroft
j, S. Riemann
as, K. Riles
c, B.P. Roe
c, L. Romero
x, A. Rosca
as, C. Rosenbleck
a, S. Rosier-Lees
d, S. Roth
a, J.A. Rubio
r, G. Ruggiero
q, H. Rykaczewski
at, A. Sakharov
at, S. Saremi
f,
S. Sarkar
al, J. Salicio
r, E. Sanchez
x, C. Schäfer
r, V. Schegelsky
ag, H. Schopper
au, D.J. Schotanus
ad, C. Sciacca
ab, L. Servoli
af, S. Shevchenko
ae, N. Shivarov
ao, V. Shoutko
m, E. Shumilov
aa, A. Shvorob
ae, D. Son
ap, C. Souga
w, P. Spillantini
q,
M. Steuer
m, D.P. Stickland
aj, B. Stoyanov
ao, A. Straessner
t, K. Sudhakar
i, G. Sultanov
ao, L.Z. Sun
u, S. Sushkov
a, H. Suter
at, J.D. Swain
j, Z. Szillasi
y,3,
X.W. Tang
g, P. Tarjan
o, L. Tauscher
e, L. Taylor
j, B. Tellili
w, D. Teyssier
w, C. Timmermans
ad, Samuel C.C. Ting
m, S.M. Ting
m, S.C. Tonwar
i, J. Tóth
l, C. Tully
aj,
K.L. Tung
gJ. Ulbricht
at, E. Valente
al, R.T. Van de Walle
ad, R. Vasquez
aq, V. Veszpremi
y, G. Vesztergombi
l, I. Vetlitsky
aa, D. Vicinanza
am, G. Viertel
at, S. Villa
ak, M. Vivargent
d, S. Vlachos
e, I. Vodopianov
y, H. Vogel
ah, H. Vogt
as, I. Vorobiev
ah,aa, A.A. Vorobyov
ag, M. Wadhwa
e, Q. Wang
adX.L. Wang
u, Z.M. Wang
u,
M. Weber
a, P. Wienemann
a, H. Wilkens
ad, S. Wynhoff
aj, L. Xia
ae, Z.Z. Xu
u, J. Yamamoto
c, B.Z. Yang
u, C.G. Yang
g, H.J. Yang
c, M. Yang
g, S.C. Yeh
aw,
An. Zalite
ag, Yu. Zalite
ag, Z.P. Zhang
u, J. Zhao
u, G.Y. Zhu
g, R.Y. Zhu
ae, H.L. Zhuang
g, A. Zichichi
h,r,s, B. Zimmermann
at, M. Zöller
aaIII. Physikalisches Institut, RWTH, D-52056 Aachen, Germany1
bNational Institute for High Energy Physics, NIKHEF, and University of Amsterdam, NL-1009 DB Amsterdam, The Netherlands cUniversity of Michigan, Ann Arbor, MI 48109, USA
dLaboratoire d’Annecy-le-Vieux de Physique des Particules, LAPP, IN2P3-CNRS, BP 110, F-74941 Annecy-le-Vieux cedex, France eInstitute of Physics, University of Basel, CH-4056 Basel, Switzerland
fLouisiana State University, Baton Rouge, LA 70803, USA gInstitute of High Energy Physics, IHEP, 100039 Beijing, China6 hUniversity of Bologna and INFN-Sezione di Bologna, I-40126 Bologna, Italy
iTata Institute of Fundamental Research, Mumbai (Bombay) 400 005, India jNortheastern University, Boston, MA 02115, USA
kInstitute of Atomic Physics and University of Bucharest, R-76900 Bucharest, Romania
lCentral Research Institute for Physics of the Hungarian Academy of Sciences, H-1525 Budapest 114, Hungary2 mMassachusetts Institute of Technology, Cambridge, MA 02139, USA
nPanjab University, Chandigarh 160 014, India oKLTE-ATOMKI, H-4010 Debrecen, Hungary3
pDepartment of Experimental Physics, University College Dublin, Belfield, Dublin 4, Ireland qINFN-Sezione di Firenze and University of Florence, I-50125 Florence, Italy rEuropean Laboratory for Particle Physics, CERN, CH-1211 Geneva 23, Switzerland
sWorld Laboratory, FBLJA Project, CH-1211 Geneva 23, Switzerland tUniversity of Geneva, CH-1211 Geneva 4, Switzerland
uChinese University of Science and Technology, USTC, Hefei, Anhui 230 029, China6 vUniversity of Lausanne, CH-1015 Lausanne, Switzerland
wInstitut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, F-69622 Villeurbanne, France xCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, E-28040 Madrid, Spain4
yFlorida Institute of Technology, Melbourne, FL 32901, USA zINFN-Sezione di Milano, I-20133 Milan, Italy
aaInstitute of Theoretical and Experimental Physics, ITEP, Moscow, Russia abINFN-Sezione di Napoli and University of Naples, I-80125 Naples, Italy
acDepartment of Physics, University of Cyprus, Nicosia, Cyprus adUniversity of Nijmegen and NIKHEF, NL-6525 ED Nijmegen, The Netherlands
aeCalifornia Institute of Technology, Pasadena, CA 91125, USA
afINFN-Sezione di Perugia and Università Degli Studi di Perugia, I-06100 Perugia, Italy agNuclear Physics Institute, St. Petersburg, Russia
ahCarnegie Mellon University, Pittsburgh, PA 15213, USA aiINFN-Sezione di Napoli and University of Potenza, I-85100 Potenza, Italy
ajPrinceton University, Princeton, NJ 08544, USA akUniversity of Californa, Riverside, CA 92521, USA
alINFN-Sezione di Roma and University of Rome, “La Sapienza”, I-00185 Rome, Italy amUniversity and INFN, Salerno, I-84100 Salerno, Italy
anUniversity of California, San Diego, CA 92093, USA
aoBulgarian Academy of Sciences, Central Lab. of Mechatronics and Instrumentation, BU-1113 Sofia, Bulgaria ap The Center for High Energy Physics, Kyungpook National University, 702-701 Taegu, Republic of Korea
aqPurdue University, West Lafayette, IN 47907, USA arPaul Scherrer Institut, PSI, CH-5232 Villigen, Switzerland
asDESY, D-15738 Zeuthen, Germany
atEidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland auUniversity of Hamburg, D-22761 Hamburg, Germany
avNational Central University, Chung-Li, Taiwan, ROC awDepartment of Physics, National Tsing Hua University, Taiwan, ROC
Received 27 August 2003; received in revised form 8 September 2003; accepted 19 September 2003
Editor: L. Rolandi
Abstract
A search for pair-produced charged Higgs bosons is performed with the L3 detector at LEP using data collected at centre-of- mass energies between 189 and 209 GeV, corresponding to an integrated luminosity of 629.4 pb−1. Decays into a charm and a strange quark or into a tau lepton and its neutrino are considered. No significant excess is observed and lower limits on the
mass of the charged Higgs boson are derived at the 95% confidence level. They vary from 76.5 to 82.7 GeV, as a function of the H±→τ νbranching ratio.
2003 Published by Elsevier B.V.
1. Introduction
In the Standard Model of the electroweak interac- tions [1] the masses of bosons and fermions are ex- plained by the Higgs mechanism [2]. This implies the existence of one doublet of complex scalar fields which, in turn, leads to a single neutral scalar Higgs boson. To date, this Higgs boson has not been di- rectly observed [3,4]. Some extensions to the Stan- dard Model contain more than one Higgs doublet [5], and predict Higgs bosons which can be lighter than the Standard Model one and accessible at LEP. In par- ticular, models with two complex Higgs doublets pre- dict two charged Higgs bosons, H±, which can be pair- produced in e+e−collisions.
The charged Higgs boson is expected to decay through H+→c¯s or H+→τ+ντ,7with a branching ratio which is a free parameter of the models. The process e+e− → H+H− gives then rise to three different signatures: c¯scs, c¯ ¯sτ−ν¯τ and τ+νττ−ν¯τ. These experimental signatures have to be disentangled from the large background of the e+e−→W+W− process, characterised by similar final states.
Data collected at centre-of-mass energies √ s = 189–209 GeV are analysed here, superseding previous results [6]. Data from √
s=130–183 GeV [7] are included to obtain the final results. Results from other LEP experiments are given in Ref. [8].
1 Supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie.
2 Supported by the Hungarian OTKA fund under contract numbers T019181, F023259 and T037350.
3 Also supported by the Hungarian OTKA fund under contract number T026178.
4 Supported also by the Comisión Interministerial de Ciencia y Tecnología.
5 Also supported by CONICET and Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina.
6 Supported by the National Natural Science Foundation of China.
7 The inclusion of the charge conjugate reactions is implied throughout this Letter.
The analyses do not depend of flavour tagging variables and are separately optimised for each of the three possible signatures.
2. Data and Monte Carlo samples
The search for pair-produced charged Higgs bosons is performed using 629.4 pb−1 of data collected in the years from 1998 to 2000 with the L3 detector [9] at LEP, at several average centre-of-mass energies, detailed in Table 1.
The charged Higgs cross section is calculated using the HZHA Monte Carlo program [10]. As an example, at√
s=206 GeV it varies from 0.28 pb for a Higgs mass,mH±, of 70 GeV to 0.17 pb formH±=80 GeV.
To optimise selections and calculate efficiencies, sam- ples of e+e−→H+H−events are generated with the PYTHIA Monte Carlo program [11] formH±between 50 and 100 GeV, in steps of 5 GeV, and between 75 and 80 GeV, in steps of 1 GeV. About 1000 events for each final state are generated at each Higgs mass.
For background studies, the following Monte Carlo generators are used: KK2f [12] for e+e−→qq(γ ),¯ e+e−→µ+µ−(γ )and e+e−→τ+τ−(γ ), BHWIDE [13] for e+e−→e+e−, PYTHIA for e+e−→ZZ and e+e−→Ze+e−, YFSWW [14] for e+e−→W+W− and PHOJET [15] and DIAG36 [16] for hadron and lepton production in two-photon interactions, respec- tively. The L3 detector response is simulated using the GEANT program [17] which takes into account the effects of energy loss, multiple scattering and show- ering in the detector. Time-dependent detector ineffi- ciencies, as monitored during the data taking period, are included in the simulations.
3. Data analysis
The analyses for all three final states are updated since our previous publications at lower centre-of- mass energies [6,7]: the searches in the H+H−→c¯scs¯
Table 1
Average centre-of-mass energies and corresponding integrated luminosities
√s(GeV) 188.6 191.6 195.5 199.5 201.7 204.9 206.4 208.0
Luminosity (pb−1) 176.8 29.8 84.2 83.3 37.2 79.0 130.8 8.3
and c¯sτ−ν¯τ channels are based on a mass depen- dent likelihood interpretation of data samples selected [18] for the studies of W pair-production, while a dis- criminant variable is introduced for the search in the τ+νττ−ν¯τ channel. These analyses are described be- low.
3.1. Search in the H+H−→c¯scs channel¯
The search in the H+H−→cs¯cs channel proceeds¯ from a selection of high multiplicity events with bal- anced transverse and longitudinal momenta and with a visible energy which is a large fraction of√
s. These criteria reject events from low-multiplicity processes like lepton pair-production, events from two-photon interactions and pair-production of W bosons where at least one boson decays into leptons. The events are forced into four jets by means of the DURHAM algorithm [19] and a neural network [18] discrimi- nates between events which are compatible with a four-jet topology and those from the large cross sec- tion e+e−→qq(γ )¯ process in which four-jet events originate from hard gluon radiation. The neural net- work inputs are the event spherocity, the energies and widths of the most and least energetic jets, the differ- ence between the energies of the second and third most energetic jets, the minimum multiplicity of calorimet- ric clusters and charged tracks for any jet, the value of they34 parameter of the DURHAM algorithm and the compatibility with energy–momentum conserva- tion in e+e− collisions. After a cut on the output of the neural network, two constrained fits are per- formed. The first four-constraint fit enforces energy and momentum conservation, modifying the jet ener- gies and directions. The second five-constraint (5C) fit imposes the additional constraint of the production of two equal mass particles. Among the three pos- sible jet pairings, the one is retained which is most compatible with this equal mass hypothesis. Events with a low probability for the fit hypotheses are re- moved from the sample and a total of 5156 events
are observed in data while 5112 are expected from Standard Model processes. The corresponding signal efficiencies are between 70% and 80%, for mH± = 60–95 GeV.
Likelihood variables [20] are built to discriminate four-jet events compatible with charged Higgs produc- tion from the dominating background from W pair- production. A different likelihood is prepared for each simulated Monte Carlo sample corresponding to a dif- ferent Higgs boson mass. Seven variables are included in the likelihoods:
• the minimum opening angle between paired jets;
• the difference between the largest and smallest jet energies;
• the difference between the di-jet masses;
• the output of the neural network for the selection of four-jet events;
• the absolute value of the cosine of the polar angle of the thrust vector;
• the cosine of the polar angle at which the positive charged8boson is produced;
• the value of the quantity 2 ln|M|, where M is the matrix element for the e+e−→W+W−→ four fermions process from the EXCALIBUR [22]
Monte Carlo program, calculated using the four- momenta of the reconstructed jets.
Fig. 1(a)–(c) shows the distributions of the last three variables while Fig. 1(d) presents the distribution of the likelihood variable formH± =70 GeV. A cut at 0.7 on this variable, which maximizes the signal sensitivity, is applied as a final selection criterion, for all mass hypotheses. The numbers of observed and expected events are given in Table 2 and the selection efficiencies in Table 3. The main contributions to the background come from hadronic W-pair decays (70%) and from the e+e−→qq(γ )¯ process (26%).
8Charge assignment is based on jet-charge techniques [21].
Fig. 1. Distributions for the H+H−→cs¯cs channel of: (a) the absolute value of the cosine of the polar angle of the thrust axis, (b) the cosine of¯ the polar angle of the positively charged boson, (c) the logarithm of the squared matrix element for the e+e−→W+W−process and (d) the selection likelihood formH±=70 GeV. The points represent the data and the open histogram the expected background. The hatched histogram indicates the expected distribution for a signal withmH±=70 GeV and Br(H±→τ ν)=0, multiplied by a factor of 10. The arrow in (d) shows the position of the cut.
Fig. 2 shows the 5C mass of the pair-produced bosons before and after the cut on the final likelihoods. Peaks from pair-production of W as well as Z bosons are visible.
3.2. Search in the H+H−→c¯sτ−ν¯τ channel
The search in the H+H−→c¯sτ−ν¯τ channel se- lects events with high multiplicity, two hadronic jets and a tau candidate. Tau candidates can be identi-
fied either as electrons or muons with momentum in- compatible with that expected for leptons originat- ing from direct semileptonic decay of W pairs, or with narrow, low multiplicity jets with at least one charged track, singled out from the hadronic back- ground with a neural network [18]. The tau energy is reconstructed by imposing four-momentum conserva- tion and enforcing the hypothesis of the production of two equal mass particles. The events must have a transverse missing momentum of at least 20 GeV
Fig. 2. Reconstructed mass spectra in the H+H−→c¯scs channel,¯ for data and expected background, for events (a) before, and (b) after, the cut on the likelihoods. The points represent the data and the open histogram the expected background. The expected distribution formH±=70 GeV and Br(H±→τ ν)=0 is shown as the hatched histogram.
and the absolute value of the cosine of the polar angle of the missing momentum is required to be less than 0.9. Finally, the di-jet invariant mass is re- quired to be less than 100 GeV and the mass recoil- ing against the di-jet system less than 130 GeV, thus selecting 1026 events in data while 979 are expected from Standard Model processes, mainly from W pair- production where one of the W bosons decays into lep-
Table 2
Number of observed data events and background expectations in the three analysis channels. The uncertainty on the background expectations is estimated to be 5%. The numbers of expected signal events formH±=70 GeV and Br(H±→τ ν)=0, 0.5 and 1 are also given for the c¯scs, c¯ ¯sτ−ν¯τ and τ+νττ−ν¯τ channels, respectively
Channel
c¯s¯cs c¯sτ−ν¯τ τ+νττ−ν¯τ
Data 2296 442 141
Background 2228 464 141
Signal 100 76 50
Table 3
Selection efficiencies for various charged Higgs masses. The effi- ciencies are largely independent of the centre-of-mass energy. The uncertainty on each efficiency is estimated to be 2%
Channel Selection efficiency (%)
mH±=60 GeV 70 GeV 80 GeV 90 GeV 95 GeV
c¯scs¯ 62 62 50 58 64
c¯sτ−ν¯τ 38 51 43 43 39
τ+νττ−ν¯τ 26 30 33 34 36
tons and the other into hadrons. The signal efficiency is about 50%.
To discriminate the signal from the background, mass dependent likelihoods [20] are built which con- tain eight variables:
• the di-jet acoplanarity;
• the angle of the tau flight direction with respect to that of its parent boson in the rest frame of the latter;
• the di-jet mass;
• the quantity 2 ln|M| calculated using the four- momenta of the reconstructed jets and tau as well as the missing momentum and energy;
• the transverse momentum of the event, normalised to√
s;
• the polar angle of the hadronic system, multiplied by the charge of the reconstructed tau;
• the sum
θ of the angles between the tau candi- date and the nearest jet and between the missing momentum and the nearest jet;
• the energy of the tau candidate, calculated in the rest frame of its parent boson and scaled by√
s.
Fig. 3. Distribution for the H+H−→csτ¯ −ν¯τ channel of: (a) the cosine of the polar angle of the hadron system multiplied by the charge of the tau candidate, (b) the sum of the angles between the tau candidate and the nearest jet and between the missing momentum and the nearest jet, (c) the scaled energy of the tau candidate in the rest frame of the parent boson and (d) the selection likelihood formH±=70 GeV.
The points represent the data and the open histogram the expected background. The hatched histogram indicates the expected distribution for mH±=70 GeV and Br(H±→τ ν)=0.5, multiplied by a factor of 5. The arrow in (d) shows the position of the cut.
The distributions of the last three variables are shown in Fig. 3(a)–(c). Fig. 3(d) presents an ex- ample of the distributions of the likelihood variable formH±=70 GeV for data, background and signal Monte Carlo. A cut at 0.6 is applied for all likeli- hoods. This cut corresponds to the largest sensitivity to a charged Higgs signal. Table 2 gives the num- bers of observed and expected events, while the se- lection efficiencies are given in Table 3. Over 95% of the background is due to W pair-production. Fig. 4
shows the reconstructed mass of the pair-produced bosons before and after the cut on the final likeli- hoods.
3.3. Search in the H+H−→τ+νττ−ν¯τchannel The signature for the leptonic decay channel is a pair of tau leptons. These are identified either via their decay into electrons or muons, or as narrow jets.
Fig. 4. Reconstructed mass spectra in the H+H−→c¯sτ−ν¯τ channel, for data and expected background, for events (a) before, and (b) after, the cut on the likelihoods. The points represent the data and the open histogram the expected background. The expected distribution formH±=70 GeV and Br(H±→τ ν)=0.5 is shown as the hatched histogram.
The selection criteria are similar to those used at lower √
s [6,7]. Low multiplicity events with large missing energy and momentum are retained. To re- duce lepton-pair background, an upper cut is placed on the value of the event collinearity angle, ξ, de- fined as the maximum angle between any pair of tracks. The distribution of this variable is shown in Fig. 5(a). The contribution from cosmic muons
is reduced by making use of information from the time-of-flight system. Fig. 5(b) presents the distri- bution of the scaled visible energy, Evis/√
s, for events on which all other selection criteria are ap- plied.
The analysis is modified with respect to those previously published [6,7] in that the normalised transverse missing momentum of the event,Pt/Evis, whose distribution is shown in Fig. 5(c), is used as a linear discriminant variable on which no cut is applied.
The efficiency of the H+H−→τ+νττ−ν¯τ selec- tion for several Higgs masses is listed in Table 3.
The numbers of observed and expected events are pre- sented in Table 2. The background is mainly formed by W-pair production (60%), two-photon interactions (26%) and lepton pair-production (9%).
4. Results
The number of selected events in each decay chan- nel is consistent with the number of events expected from Standard Model processes. A technique based on a log-likelihood ratio [4] is used to calculate a confi- dence level (CL) that the observed events are consis- tent with background expectations. For the c¯scs and¯ c¯sτ−ν¯τchannels, the reconstructed mass distributions, shown in Figs. 2(b) and 4(b), are used in the calcula- tion, whereas for theτ+νττ−ν¯τ channel, the distribu- tion of the normalised transverse missing momentum, shown in Fig. 5(c), is used.
The systematic uncertainties on the background level and the signal efficiencies are included in the confidence level calculation. These are due to finite Monte Carlo statistics and to the uncertainty on the background normalisation. The former uncertainty is 5% for the background and 2% for the signal Monte Carlo samples. The uncertainty on the background normalisation is 3% for the H+H− → c¯scs chan-¯ nel and 2% for the csτ¯ −ν¯τ and τ+νττ−ν¯τ chan- nels. The systematic uncertainty on the signal effi- ciency due to the selection procedure is estimated by varying the selection criteria and is found to be less than 1%. These systematic uncertainties decrease the mH± sensitivity of the combined analysis by about 200 MeV.
Fig. 6 compares the resulting background confi- dence level, 1−CLb, for the data to the expectation in
Fig. 5. Distribution for the H+H−→τ+νττ−ν¯τ channel of: (a) the event collinearity angle,ξ, (b) the scaled visible energy and (c) the normalised transverse missing momentum of the event. In (a) and (b) all other selection criteria are applied and the arrows indicate the cut on the displayed variable. The points represent the data and the open histogram the expected background. The hatched histograms indicate the expected signal distributions formH±=70 GeV and Br(H±→τ ν)=1.
the absence of a signal, for three values of the H±→ τ νbranching ratio: Br(H±→τ ν)=0, 0.5 and 1. The 68.3% and 95.4% probability bands expected in the absence of a signal are also displayed and denoted as 1σ and 2σ, respectively. A slight excess of data ap- pears aroundmH±=69 GeV for Br(H±→τ ν)=0, as previously observed [6]. It is compatible with a 2.5σ upward fluctuation in the background. The ex- cess is also compatible with a 2.9σdownward fluctua-
tion of the signal.9As observed in Fig. 6(b) and (c), no excess is present in the csτ¯ −ν¯τ andτ+νττ−ν¯τ chan- nels aroundmH±=69 GeV. Therefore, the c¯scs excess¯ is interpreted as a statistical fluctuation in the back- ground and lower limits at the 95% CL on mH± are
9As an example, for Br(H±→τ ν)=0.1, these figures are 1.8σ and 2.7σ, respectively.
Fig. 6. The background confidence level, 1−CLb, as a function ofmH±for the data (solid line) and for the expectation in the absence of a signal (dashed line), for three values of the H±→τ νbranching ratio. The shaded areas represent the symmetric 1σand 2σ probability bands expected in the absence of a signal.
derived [4] as a function of Br(H±→τ ν). Data at
√s=130–183 GeV [7] are included to obtain the lim- its. Fig. 7 shows the excludedmH± regions for each of the final states and their combination, as a function of Br(H±→τ ν). Table 4 gives the observed and the median expected lower limits for several values of the branching ratio.
In conclusion, refined analyses and larger centre- of-mass energies improve the sensitivity of the search for charged Higgs bosons produced in e+e−collisions as compared to previous results [6,7]. No significant
Table 4
Observed and expected lower limits at 95% CL for different values of the H±→τ νbranching ratio. The minimum observed limit is at Br(H±→τ ν)=0.26
Br(H±→τ ν) Lower limits (GeV) at 95% CL
observed expected
0.0 76.7 77.5
0.26 76.5 75.6
0.5 76.6 76.5
1.0 82.7 84.6
Fig. 7. Excluded regions for the charged Higgs boson in the plane of the H±→τ ν branching fraction versus mass, for the analyses of each final state and their combination. The dashed line indicates the median expected limit in the absence of a signal. Regions below mH±=50 GeV are excluded by data collected at the Z resonance [23] and at√
s=130–183 GeV [7].
excess is observed in data and a lower limit at 95% CL on the charged Higgs boson mass is obtained as mH±>76.5 GeV,
independent of its branching ratio.
References
[1] S.L. Glashow, Nucl. Phys. 22 (1961) 579;
S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264;
A. Salam, in: N. Svartholm (Ed.), Elementary Particle Theory, Almqvist & Wiksell, Stockholm, 1968, p. 367.
[2] P.W. Higgs, Phys. Lett. 12 (1964) 132;
P.W. Higgs, Phys. Rev. Lett. 13 (1964) 508;
P.W. Higgs, Phys. Rev. 145 (1966) 1156;
F. Englert, R. Brout, Phys. Rev. Lett. 13 (1964) 321;
G.S. Guralnik, C.R. Hagen, T.W.B. Kibble, Phys. Rev. Lett. 13 (1964) 585.
[3] L3 Collaboration, P. Achard, et al., Phys. Lett. B 517 (2001) 319.
[4] ALEPH, DELPHI, L3 and OPAL Collaborations, LEP Work- ing Group for Higgs Boson Searches, Phys. Lett. B 565 (2003) 61.
[5] S. Dawson, et al., The Physics of the Higgs Bosons: Higgs Hunter’s Guide, Addison–Wesley, Menlo Park, CA, 1989.
[6] L3 Collaboration, M. Acciarri, et al., Phys. Lett. B 466 (1999) 71;
L3 Collaboration, M. Acciarri, et al., Phys. Lett. B 496 (2000) 34.
[7] L3 Collaboration, M. Acciarri, et al., Phys. Lett. B 446 (1999) 368.
[8] ALEPH Collaboration, A. Heister, et al., Phys. Lett. B 543 (2002) 1;
DELPHI Collaboration, J. Abdallah, et al., Phys. Lett. B 525 (2002) 17;
OPAL Collaboration, G. Abbiendi, et al., Eur. Phys. J. C 7 (1999) 407.
[9] L3 Collaboration, B. Adeva, et al., Nucl. Instrum. Methods A 289 (1990) 35;
J.A. Bakken, et al., Nucl. Instrum. Methods A 275 (1989) 81;
O. Adriani, et al., Nucl. Instrum. Methods A 302 (1991) 53;
B. Adeva, et al., Nucl. Instrum. Methods A 323 (1992) 109;
K. Deiters, et al., Nucl. Instrum. Methods A 323 (1992) 162;
M. Chemarin, et al., Nucl. Instrum. Methods A 349 (1994) 345;
M. Acciarri, et al., Nucl. Instrum. Methods A 351 (1994) 300;
G. Basti, et al., Nucl. Instrum. Methods A 374 (1996) 293;
A. Adam, et al., Nucl. Instrum. Methods A 383 (1996) 342;
O. Adriani, et al., Phys. Rep. 236 (1993) 1.
[10] HZHA version 2 is used;
P. Janot, in: G. Altarelli, T. Sjöstrand, F. Zwirner (Eds.), Physics at LEP2, Vol. 2, CERN 96-01, 1996, p. 309.
[11] PYTHIA version 5.722 is used;
T. Sjöstrand, preprint CERN-TH/7112/93 (1993), revised 1995;
T. Sjöstrand, Comput. Phys. Commun. 82 (1994) 74.
[12] KK2f version 4.14 is used;
S. Jadach, B.F.L. Ward, Z. W¸as, Comput. Phys. Commun. 130 (2000) 260.
[13] BHWIDE version 1.03 is used;
S. Jadach, W. Placzek, B.F.L. Ward, Phys. Lett. B 390 (1997) 298.
[14] YFSWW version 1.14 is used;
S. Jadach, et al., Phys. Rev. D 54 (1996) 5434;
S. Jadach, et al., Phys. Lett. B 417 (1998) 326;
S. Jadach, et al., Phys. Rev. D 61 (2000) 113010;
S. Jadach, et al., Phys. Rev. D 65 (2002) 093010.
[15] PHOJET version 1.05 is used;
R. Engel, Z. Phys. C 66 (1995) 203;
R. Engel, J. Ranft, Phys. Rev. D 54 (1996) 4244.
[16] DIAG 36 Monte Carlo;
F.A. Berends, P.H. Daverfeldt, R. Kleiss, Nucl. Phys. B 253 (1985) 441.
[17] GEANT version 3.15 is used;
R. Brun, et al., preprint CERN DD/EE/84-1 (1985), revised 1987;
The GHEISHA program (H. Fesefeldt, RWTH Aachen Report PITHA 85/02, 1985) is used to simulate hadronic interactions.
[18] L3 Collaboration, P. Achard, et al., Measurement of the cross section of W pair-production at LEP, in preparation.
[19] S. Bethke, et al., Nucl. Phys. B 370 (1992) 390.
[20] L3 Collaboration, P. Achard, et al., Phys. Lett. B 545 (2002) 30.
[21] L3 Collaboration, M. Acciarri, et al., Phys. Lett. B 413 (1997) 176.
[22] EXCALIBUR version 1.11 is used;
F.A. Berends, R. Kleiss, R. Pittau, Comput. Phys. Commun. 85 (1995) 437.
[23] L3 Collaboration, O. Adriani, et al., Phys. Lett. B 294 (1992) 457;
L3 Collaboration, O. Adriani, et al., Z. Phys. C 57 (1993) 355.
